1. Field of the Invention
This invention relates to techniques for protection of ships from hazardous underwater water objects including mines and torpedoes. This invention particularly relates to techniques for scanning for such hazardous objects with laser beams.
2. Description of the Prior Art
Conventional techniques for detecting for hazardous objects at sea have limited detection range and therefore do not provide sufficiently early warning of the presence of hazards to permit adequate defensive measures to be taken. Conventional techniques often require the use of elevated devices separated from the ship, such as aircraft, kites, balloons or other elevated platforms to extend the range for detection.
Some laser beam scanning techniques have been developed for underwater hazard detection which use low angle pulsed laser beams, but these suffer the difficulties associated with short-pulse, very high repetition rate lasers using pulsed gating for scanning.
In U.S. Pat. No. 4,690,551, LASER RADAR UTILIZING PULSE TONE WAVEFORM, Edwards et al. disclose a laser radar system including both a pulsed laser and a CW laser. An acousto-optic angular multiplexer is used to provide a frequency and angle shifted beam. A scanner and pointing mirror are used to move the beam at the target.
In U.S. Pat. No. 4,326,799, ACTIVE-PASSIVE SCANNING SYSTEM. Keene et al. disclose a scanning system for use on an aircraft platform which, in the active mode, scans a laser beam with a rotating wedge in a downward looking Palmer pattern.
In U.S. Pat. No. 4,755,818, AIRCRAFT COLLISION WARNING SYSTEM, Conrad discloses an aircraft warning collision system which uses a low power pulsed laser system to project narrow bandwidth radiation in a circular field about the aircraft to detect airborne hazards.
In U.S. Pat. No. 4,324,491, DUAL MODE GUIDANCE SYSTEM. Hueber discloses a laser scanning system for aircraft which utilizes a circular scan.
In U.S. Pat. No. 4,290,043, METHOD OF AND SYSTEM FOR DETECTING MARINE OBSTACLES, Kaplan discloses a marine obstacle avoidance system including an optical transmitter and receiver. An optical beam is transmitted from the ship at a low angle of incidence to the water line and reflections active an alarm.
Such conventional approaches do not adequately provide detection at a sufficient standoff distance from the ship, nor sufficient identification of the hazard, to locate and track the hazard with the precision required by defensive devices to ensure termination of the hazard.